Memory devices for storing digital data are abundant in today's computers, automobiles, cellular telephones and media information cards. Certain of these memory devices or storage elements, referred to as non-volatile memory, retain the stored digital data when power is removed from the device. For example non-volatile memory instructions instruct a computer during the boot-up process and store instructions and data for sending and receiving calls in a cellular telephone. Electronic products of all types, from microwave ovens to heavy industrial machinery, store their operating instructions in non-volatile storage elements. Certain non-volatile memory devices offer multiple programming capabilities, with previously stored information overwritten by new data. Other non-volatile devices provide only one-time programmability.
Volatile memory devices, another class of memory devices, loose the stored information when power is removed. Dynamic random access memories (DRAM) and static random access memories (SRAM) are two types of volatile storage elements.
A read-only memory (ROM) is one type of permanent data storage non-volatile memory. Once stored in the ROM device, the data cannot be overwritten or otherwise altered. The ROM is “programmed” during manufacture by making permanent electrical connections in selected memory cells. The stored information can be changed only by redesigning the ROM.
A programmable read-only memory (PROM) is a non-volatile device that can be programmed only once, either during fabrication or thereafter. In one PROM embodiment each memory cell comprises a fusible link The PROM is “programmed” by opening or blowing a fusible link in selected cells, while other links remain intact. Advantageously, manufacturers can offer a single PROM hardware design that can be programmed by the purchaser.
An erasable programmable read-only memory (EPROM), another non-volatile memory device, can be erased and reprogrammed as desired. The EPROM is programmed electronically and erased by passing ultraviolet light through an ultraviolet-permeable quartz window formed in the package. An EEPROM (electronically erasable programmable read-only memory) can be programmed, electronically erased and reprogrammed.
A flash EEPROM memory is a type of EEPROM non-volatile memory that is especially prevalent in electronic devices where the user desires to add or change information after the memory device has been fabricated and inserted into the electronic device. For example, flash memory allows the user to add addresses and calendar entries in a personal digital assistant and erase and re-use media cards that store pictures taken with a digital camera. Flash memory devices differ form other EEPROM devices in that a flash memory permits block of stored data words to be simultaneously erased, whereas other EEPROM devices permit the erasure of only single words. Erasing a memory block in a non-flash EEPROM is a much slower process than the same operation in a flash memory. Also, a flash EEPROM is typically smaller than other types of EEPROM memory devices. A flash memory stores data bits in a floating polysilicon gate and a tunnel oxide immediately below the control gate/gate oxide stack of the conventional MOSFET.
An anti-fuse device is yet another PROM non-volatile memory. Unlike the PROM device where the fusible links are formed in a closed state, the anti-fuse is formed in an opened state and programmed to a closed state using a voltage that is higher than the normal operating supply voltage for the integrated circuit on which the anti-fuse is formed. Transistors in the anti-fuse programming circuitry must be fabricated with higher junction breakdown voltages than the conventional transistor. But higher doping levels in advanced integrated circuit devices result in lower junction breakdown voltages, rendering anti-fuse devices less compatible with these advanced circuits. Also, certain of the anti-fuse materials are not compatible with standard CMOS fabrication processes.
Certain non-volatile memory devices are referred to as “one-time programmable (OTP),” memories, including anti-fuse devices, EPROM's and PROM's. OTP memory can be further subdivided into those with relatively large arrays of storage elements (or cells) and those with a relatively small number of cells. OTP devices with few cells are useful for trimming analog device parameters (e. g., fusible links are disposed to short out or add individual resistors) and for permanently storing a relatively small number of non-modifiable data bits. For example, an integrated circuit chip can be identified by unique data bits stored in an on-chip OTP device. The identification information can be used to track the chip during service and associate the chip with a wafer from which it was singulated.
Such OTP non-volatile memory devices can comprise conductive fuses or fusible links disposed in a conductive interconnect structure of the integrated circuit chip. Depending upon the process technology selected, a material of the conductive layer comprises polysilicon, metal or a silicide material. The OTP devices can be formed on a top layer or lower layers of the interconnect structure. Generally, the fusible link is formed coincident with the formation of the interconnect structure by adding fusible link features to the interconnect structure mask, followed by patterning and etching steps to form the interconnect structure and the conductive fusible links.
To program (i.e., open) the fusible links according to one technique, the integrated circuit is masked to expose the fusible links to be opened. Laser energy directed at the exposed fusible links opens therin. The resulting combination of opened and closed fusible links represents information, such as a chip identification code, that can be retrieved by reading the pattern of opened and closed links.
A “few times programmable” (FTP) memory device comprises a capacitor disposed above and connected in series with the gate of a metal-oxide field effect transistor (MOSFET). Charge stored on the capacitor turns the MOSFET on. A MOSFET without stored charged on the series capacitor remains off. Thus a stored one or zero bit is determined from the state of the MOSFET. The structure is designed to retain the charge for years, although such devices can be reprogrammed.
Disadvantageously, according to the prior art, the formation of fusible links in an integrated circuit device requires the creation of process masks and the execution of additional process steps. Each mask step adds cost to the fabrication process and increases the probability of a circuit defect. Also, the fusible links consume chip area that could otherwise be devoted to active devices.